Efforts to improve upon the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for at least fifty years and have led to the relatively recent market introduction of a number of superior polyalpha-olefin synthetic lubricants, primarily based on the oligomerization of alpha-olefins or 1-alkenes. In terms of lubricant property improvement, the thrust of the industrial research effort on synthetic lubricants has been toward fluids exhibiting useful viscosities over a wide range of temperature, i.e., improved Viscosity Index, while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. To match synthetic lubricants with their intended application, these lubricants have, in the past, been formulated for specific properties. For example, poly-alpha-olefins (PA) were produced from 1-decene polymerization over promoted BF.sub.3 or AlCl.sub.3 catalysts. PAOs which have usually very high VI and very low pour points and low traction coefficients are particularly desirable lubricants because they not only function effectively over a broad range of temperatures, but are also highly energy efficient. Synthetic fluids with high traction coefficients have, in the past, been considered to be a different group of synthetic fluids. Traction coefficients are usually dependent on the chemical compositions of the fluid, and the literature discloses a correlation between traction coefficient and pressure-viscosity coefficient, which is also related to oil film thickness. M. Muraki and Y. Kimura, Traction Characteristics of Synthetic Hydrocarbon Oils, 7 Journal of JSLE 119 (International Edition, 1986); Mobil EHL Guidebook, 3rd Edition, Commercial Marketing, Technical Publications, 3225 Gallows Road, Fairfax, Va. 22037. Generally, fluids with higher traction coefficient generate thicker films, which means better protection of machine parts and less wear and reduced metal fatigue. Fluids with high traction coefficients not only offer better lubricating protection, but also are useful as traction fluids, providing high power transmission at low slip.
Mechanisms such as automotive transmissions, automotive power steering pumps, shock absorbers, industrial hydraulic systems, and gear reducers contain power transmission fluids. These fluids must have relatively high resistance to shear, but must also be relatively noncorrosive toward the materials of construction used in the power transmission mechanism. Further, the fluids must resist degradation (e.g., oxidation) over extended periods of use. To meet these rigorous criteria, industry has turned to synthetic power transmission fluids, examples of which include Santotrac (a registered trademark of the Monsanto Corporation) and the Delvac 1 and Mobil 1 power transmission fluids (Delvac 1 and Mobil 1 are registered trademarks of Mobil Oil Corporation).
Viscosity Index (VI) is the most common measure that is applied to the decrease in viscosity of petroleum oils with increasing temperature. A series of Pennsylvania oils exhibiting relatively small change in viscosity with changing temperature is arbitrarily assigned a VI of 100,whereas a series of Gulf Cost oils whose viscosities change relatively greatly is assigned a VI of 0. From the viscosity measurements at 40.degree. and 100.degree. C., the VI of any oil sample can be obtained from detailed tables published by the ASTM (ASTM D-2270) 14 Kirk-Othmer Encyclopedia of Chemical Technology 489 (Wiley, 1981). U.S. Pat. No. 4,913,794 to Le et al. teaches a method for improving the Viscosity Index of a lubricant stock and is incorporated herein by reference for its discussions of Viscosity Index and lubricant upgrading processes.
U.S. Pat. No. 3,671,598 to Moore relates to the isomerization of adamantane-containing compounds in the presence of sulfuric acid to provide useful traction fluids.
U.S. Pat. No. 3,648,531 to Duling et al. teaches a traction drive containing a fluid comprising an alkyladamantane dimer or an alkyladamantanol dimer.
U.S. Pat. No. 3,671,600 to Moore teaches ethylation of an adamantane nucleus in the presence of a strong acid and BF.sub.3 etherate.
U.S. Pat. No. 3,793,203 to Driscoll et al. discloses polyolefins, paraffins, and polar compounds containing a gem-dialkyl substituted back-bone structure which are useful as traction fluids.
U.S. Pat. No. 3,903,001 to Gates et al. and 4,180,466 to Newingham et al. teach synthetic lubricants for limited slip differentials.
U.S. Pat. No. 3,966,624 to Driscoll et al. relates to a hydrogenated polymeric traction fluid containing a light olefin oligomer and at least one saturated adamantane compound.
Oxygenated compounds from polyisobutylene (e.g., ketones, esters, and alcohols) useful as traction fluid components are taught in U.S. Pat. No. 3,972,941 to Driscoll et al. Example 6 at column 7 notes that the additives of the '914 patent can be blended with paraffinic or naphthenic lubricants or with synthetic naphthenes or adamantanes.
U.S. Pat. No. 3,972,243 to Driscoll et al. relates to polyolefins, paraffins, and polar compounds containing a gem-structured backbone structure which are useful as lubricant additives and traction fluid components.
The completely hydrogenated dimers of alpha-methylstyrene as disclosed in U.S. Pat. No. 3,994,816 to Wygant illustrate the well-known problem of low Viscosity Index in power transmission fluids. The temperature/viscosity data shown at column 6 for 2,4-dicyclohexyl-2-methylpentane containing 3% cyclic dimer indicate a Viscosity Index of about 9.
Traction fluids containing compounds having two adamantane nuclei linked through an alkylene radical are disclosed in U.S. Pat. No. 4,008,251 to Moore et al.
Fluorinated adamantanes have also been found to be useful traction fluids, as shown in U.S. Pat. No. 4,041,086 to Moore et al. Ethers of adamantane have similarly been found to be useful as traction fluids, as shown in U.S. Pat. No. 4,043,927 to Duling et al.
U.S. Pat. No. 4,329,529 to Nambu teaches a traction fluid produced by hydrogenating the alkylation product of styrene with xylene and/or toluene, and optionally with ethylbenzene.
U.S. Pat. No. 4,371,726 to Horita et al. relates to a traction fluid containing three substituted cycloalkane rings, while U.S. Pat. No. 4,675,459 to Yuasa et al. teaches a traction drive fluid comprising a base stock which contains compounds having fused cyclohexane and norbornene rings.
Similarly, U.S. Pat. No. 4,751,335 to Kubo et al. teaches a method for making a traction fluid by decomposing a compound having four aromatic rings.
Traction fluids comprising cis- and trans-perhydroacenaphthene are taught in U.S. Pat. Nos. 4,761,510 and 4,783,565 to Naruse et al.
Ester compounds containing one or two cyclic or bicyclic alkanes which are useful as traction fluids are disclosed in U.S. Pat. No. 4,786,427 to Dare-Edwards.
U.S. Pat. No. 4,889,649 to Murai et al. teaches a oxidatively stable traction fluid containing 2,4-dicyclohexyl-2-methylpentane, polycyclohexlalkane, and a perhydroindane derivative.
U.S. Pat. No. 5,043,503 to Del Rossi et al. discloses alkylated polycycloparaffinic compounds useful as lubricating stocks which are prepared by alkylating a polycycloparaffinic compound in the presence of a catalyst having a Constraint Index of from about 1 to about 10.
U.S. Pat. No. 5,053,568 to Chen teaches a lubricant additive and composition comprising the copolymer of 1-vinyladamantane and a 1-alkene having from about 4 to about 16 carbon atoms, wherein the copolymer has a Viscosity Index of at least about 80 and a kinematic viscosity of at least about 6 cS at 212.degree. F.
U.S. Pat. No. 5,085,792 to Narihiko et al. relates to a synthetic traction fluid comprising two substituted cyclohexane nuclei connected through an ester linkage.
U.S. Pat. No. 5,107,041 to Abe et al. relates to a synthetic traction fluid derived from a 1,1-dicyclohexyl cycloalkane.
Many of the fluids described in the above patents have high traction coefficients, but typically have very low VI. For example, the hydrogenated dimer of alpha-methyl-styrene (2,4-dicyclohexyl-2-methylpentane) has a VI of only 9. Known adamantane-based traction fluids (discussed in U.S. Pat. Nos. 3,966,624, 3,648,531, 4,043,927, 4,008,251, 3,994,816, and 4,889,649) have high traction coefficients but very low Viscosity Indices and tend to lose their viscosities quickly as the operating temperatures increase. When the fluid loses its viscosity, it also loses its lubricating film and thus its protection capability. For these reasons, it would be beneficial to provide a fluid having a high traction coefficient and also high VI and low pour point, which would lubricate effectively over a wide operating temperature range.
Meeting the competing requirements of stability, lubricity, traction coefficient, good low temperature properties, and noncorrosivity has, in the past, required mixing an additive package of specialty chemicals with the base stock. Thus it would be desirable to provide a base stock which itself has a high Viscosity Index and low pour point.